> In short, shouldn't the amount of money you expect for a fine be at least vaguely proportional to the harms caused?

No. Fines are punitive and intended for deterrence. You're thinking of reimbursements (cost recovery and damages). The latter is intended to be proportional to real world harm/costs, the former need not be.

A traffic law violation could cause no harm, but you'll still get fined for it.

Some jurisdictions even impose fines based on how deep the defendants' wallet is, calculating the amount that might serve as appropriate deterrent/punishment based on their financial situation. Fining a billionare $50,000 for literring will still not serve as a deterrent, but doing the same to a minimum wage worker would unfairly financially ruin them.